1. Field of the Invention
The present invention relates in general to a tracking servo system of a compact disk or digital video disk in a compact disk player (referred to hereinafter as CDP) or digital video disk player (referred to hereinafter as DVDP), and more particularly to a tracking error detector employing a phase difference detection method, in which a tracking error of the DVD is detected on the basis of the phase difference detection method.
2. Description of the Prior Art
Recently, the social structure has been developed according to a high-level information technique. For example, a video cassette recorder (VCR) or audio system has been used to reproduce video or audio data using a video or audio tape as auxiliary storage means. However, recently, for the purpose of providing video and audio of larger amount and higher definition, a DVDP or CDP is actively used to reproduce video or audio data using a DVD or CD as auxiliary storage means of small size and large capacity.
FIG. 1 is a functional block diagram illustrating the construction of a general CDP or DVDP. As shown in this drawing, the CDP or DVDP comprises a CD or DVD 1 which is auxiliary storage means for storing data of contents desired by the user, a spindle motor 2 for rotating the CD or DVD 1 at a constant speed, a pick-up 3 for scanning a laser beam on the disk 1 to read data recorded thereon, a sled motor 4 for moving the pick-up 3 vertically with respect to a rotating direction of the disk 1, a high-frequency amplifier 5 for amplifying a laser signal detected by the pick-up 3, a servo processor 6 for analyzing an output signal from the high-frequency amplifier 5 to discriminate whether the pick-up 3 accurately traces a track of the disk 1, and a digital signal processor 7 for converting the output signal from the high-frequency amplifier 5 into data processible in an application logic block and outputting the converted data to the servo processor 6. The digital signal processor 7 also outputs desired data to a speaker.
The CDP or DVDP further comprises a driver 8 for controlling speeds of the spindle and sled motors 2 and 4 in response to disk track data from the servo processor 6 and digital signal processor 7, and a microprocessor 9 for controlling the entire operation of the CDP or DVDP.
The operation of the CDP or DVDP with the above-mentioned construction will hereinafter be described.
When the disk 1 is rotated at a constant speed by the spindle motor 2, infrared rays are emitted and reflected on the surface of the disk 1 and current based on the reflected amount is read by the pick-up 3. The read current from the pick-up 3 is amplified by the high-frequency amplifier 5 and then applied to the servo processor 6. The servo processor 6 analyzes the output signal from the high-frequency amplifier 5 to discriminate whether the pick-up 3 accurately traces the track of the disk 1. The driver 8 controls the motor speed in accordance with the result discriminated by the servo processor 6. The output signal from the high-frequency amplifier 5 is also applied to the digital signal processor 7, so that it can be converted into data processible in the application logic block. The driver 8 controls the spindle motor 2 in response to output data from the digital signal processor 7. The above operation is controlled by the microprocessor 9.
Noticeably, in an apparatus capable of reproducing both the DVD and CD, a grating lens (not shown) in the pick-up 3 must be adjusted according to a track pitch of the inserted disk to detect a tracking error. At this time, once the grating lens is fixed, only the disk of one type can be reproduced, which is called a 3-beam method. For this reason, it is impossible to perform the tracking servo operation by detecting tracking errors of the CD and DVD with different track pitches in the same manner.
On the other hand, a grating adjustment apparatus of a 3-beam type optical pick-up is disclosed in U.S. Pat. No. 4,754,129, filed in the name of Samsung Electronics Co. Ltd, Rep. of Korea. The grating adjustment apparatus is used in assembling of the 3-beam type optical pick-up, for automatically and readily adjusting a grating angle to obtain a tracking error signal which is larger than target values of sub-laser beams scanned on the same track of a CD. The maximum value can automatically be adjusted within the range of a tracking error, which is detected on the basis of the grating adjustment by an indicator. The grating adjustment apparatus comprises transducing amplifiers for converting sub-beam current signals from the optical pick-up into voltage signals, a differential amplifier for performing a differential amplification operation with output signals from the transducing amplifiers, a band pass filter for removing a noise from an output signal from the differential amplifier, a low frequency lag compensator for doing lead compensation for a high frequency of an output signal from the band pass filter and restricting a bandwidth of an open loop gain of the apparatus, a differentiator for generating a derivative variable of an output signal from the low frequency lag compensator, a discriminator for discriminating the level of an output signal from the differentiator, a system controller for controlling the system operation in response to an output signal from the discriminator, a motor driving stage for driving a motor under the control of the system controller, a jig actuator for actuating a jig according to a rotational torque of the motor, and an indicator for indicating a system state under the control of the system controller.
In particular, the grating adjustment apparatus is adapted to automatically adjust the grating angle to obtain a target value of the tracking error signal. The grating angle is adjusted on the basis of the tracking error signal detected by the discriminator in such a manner that the target value of the tracking error signal can be maintained to drive the motor. However, the above-mentioned grating adjustment apparatus is limited to the function of merely detecting and maintaining the tracking error.